1. Field of the Invention
The present invention relates to the field of the hypersonic laminar flow control and particularly to laminar flow control on hypersonic vehicle surfaces.
2. Description of the Related Art
In many practical cases, laminar-turbulent transition is due to amplification of unstable modes in the boundary layer. At hypersonic speeds, boundary layer instability relates to the second mode which is the result of an inviscid instability present due to a region of supersonic mean flow relative to the disturbance phase velocity. It is associated with the family of trapped acoustic waves propagating in a waveguide between the wall and the local sonic line. The second mode induces pressure disturbances of frequency&gt;50 kHz belonging to the ultrasonic band. The delay in laminar-turbulent transition can lead to substantial performance gains for hypersonic aircraft and space vehicles. A method is needed to provide cost effective, passive and easily integratable delay of such laminar and turbulent transition.
U.S. Pat. No. 4,802,642 entitled "Control of Laminar Flow in Fluids by Means of Acoustic Energy" provides a method for laminar flow control involving infusion of the acoustic energy into the boundary layer. This active method of laminar flow control requires a complicated system for acoustic generators and its control to achieve stabilization of Tollmien-Schlichting waves. It is difficult to use such a method for hypersonic vehicles due to high levels of thermal loads on their skin.
A patent search has also revealed the following patents:
U.S. Pat. No. 4,392,624, entitled "Implanted Boundary Layer Trip" involves an improved technique for controlling the ablative symmetry of a re-entry vehicle nose. Carbon-carbon material is placed relatively far upstream from the boundary layer instability region and cannot be used for laminar flow control.
U.S. Pat. No.'s 4,907,765, 5,907,765, and 5,069,403 provide drag reduction on surfaces with turbulent flows. Conformal drag reduction articles have surfaces with a riblet configuration which reduces turbulent friction. Such articles (films, fiber composite materials, etc.) are not effective for laminar flow control. Surface riblets produced by these films do not absorb disturbance energy from a laminar boundary layer. Riblets could trip laminar flow rather than stabilize it. Due to high thermal loads on turbulent hypersonic surfaces, it is very difficult to utilize the methods described in these patents for hypersonic vehicles.
U.S. Pat. No. 5,167,387 entitled "Porous Airfoil and Process" discloses a process for making airfoils self-adaptive to dissimilar flow conditions. It is attained by placing cavities with contoured barrier walls beneath a porous upper and lower surface patch that stretches over the nominal chord of an airfoil. These cavities permit the high pressures occurring in the nose and trailing edge regions to be vented toward the zones of relatively low pressure in the mid-section of the airfoil. This porous configuration increases instability of laminar flow due to blowing. This airfoil device can cause tripping of the boundary layer rather than its laminarization.